How many people test the hardness of their water? Most people can get their head around ammonia, nitrite, nitrate and pH. However, the hardness is a little bit different. I actually don't really bother with it, never had a problem. My water isn't too soft so that's why.

I never test the hardness of my water. Oscars can live in pretty much all the ranges of hardness of water in the uk (and the range of Ph too). Just make sure that any rocks added dont leech into the water. Test this by putting some vinegar on the rock, if it fizzes then dont use it

My discus might have some trouble as I live in a hard water area but my RO machine sorts that out

My water is extremely hard.(GH = 26+ degrees) I've never messed with that because that's just how my local water supply is. My fish have never seemed to care. General Hardness, at least as far as I've seen, isn't too important.

My carbonate hardness (KH) is extremely low. It sometimes is undetectable coming out of the tap. In those cases I will add some back into it for pH stability. Without it I've seen my pH drop from neutral to 5.5 overnight. Especially when adding feeders. You've got to have some KH to balance out the CO2 produced by your fishes. But you don't need much in my experience. If you're above 2 degrees your fine for South and Central American fish. The more KH the more likely your water will remain stable at a higher pH.

German Degrees to PPM conversion

(degrees hardness) x 17.9 = PPM